This type of escapement is known and described in the work by Paul M. Chamberlain entitled “It's About Time”, published in London in 1978 by The Holland Press. The author describes a tangential impulse escapement by the Brothers Melly at page 79. FIG. 1 of this description shows a drawing of this prior art.
The escapement in question includes a lever device B hinged on a pivot. The top arm of this lever device is formed by a lever ending in a fork for cooperating with an impulse pin L of a roller A. The bottom arm includes a ring surrounding an escape wheel. The ring carries four pallet-stones, namely two impulse pallet-stones R and R′ for imparting an impulse to roller A and two locking pallet-stones D and D′ for locking the escape wheel, the latter including teeth E, F, G, H cooperating with the pallet-stones of the ring.
FIG. 1 shows an operating phase of the escapement of the prior art. Preceding this phase, tooth F of the wheel was locking against locking pallet-stone D. Roller A, rotating in the direction of arrow C has released the escape wheel, which then rotates in the direction of arrow P. Tooth E of the wheel enters into contact with pallet-stone R of the ring, which causes lever device B to rotate in the direction of arrow Q and imparts an impulse to the roller by impulse pin L in the direction of arrow C. Once this impulse has ended, the wheel and more specifically the tooth H thereof is locked by face D′. From this moment, the roller travels through its additional arc and reverses its direction. Impulse pin L then drives the fork in the opposite direction and the lever device rotates in the opposite direction to that shown by arrow Q. The locking of tooth H on impulse pallet-stone D′ is interrupted, which enables tooth G of the wheel to impart another impulse to the ring via the impulse pallet-stone R′ thereof and to send roller A back again.
The explanations given above show that this is a tangential impulse escapement. Indeed, the impulse is imparted by the top of the tooth of the escape wheel onto a point of the pallet-stone that remains the same during the entire duration of the impulse, like the teeth of a gear. There is thus no or very little sliding of the tooth on the pallet-stone, which is not the case of a lever escapement, for example. The tangential impulse escapement thus does not involve any lubrication system, which is very fragile, prone to aging and currently used in lever escapements. It will be noted however that a single lubrication is carried out in order to prevent any wear phenomenon of the members in contact with each other.
It will also be indicated that a tangential impulse escapement was described in the work of G. Daniels entitled: “La montre: Principles et Méthodes de Fabrication” at pages 249 to 252, Scriptar Editions S. A., La Conversion/Lausanne, 1993. This escapement, called a coaxial escapement, includes two impulse-receiving pallet-stones, one located on the lever, the other directly associated with the roller. It may be considered that the present invention simplifies matters by associating the two impulse pallet-stones with a single lever.
Returning now to the Brothers Melly escapement shown in FIG. 1, it will be seen that it is affected by at least two drawbacks: its space requirement and operating reliability, which does not seem sure.
FIG. 1 shows that the space requirement of the Brothers Melly system is difficult to reconcile with use in a watch of normal size. The length of the system would be difficult to incorporate in a timepiece worn on the wrist.
In the same Figure, two angles are drawn: a first angle α showing the angle of displacement of impulse pallet-stone R for the duration of the impulse and a second angle β showing the angle of displacement of the tip of tooth E for the same impulse duration. We have: α=2° and β=8°. In other words, the impulse that the impulse pallet-stone receives from the tooth by is only effective over an excursion of 2 degrees of the lever and one might wonder whether such a short angle can transmit all of the energy deployed by the escape wheel to the roller. Moreover, this very small angle leaves no security margin, given the inherent manufacturing tolerances of the mechanical parts involved here. The situation is however slightly better as regards impulse pallet-stone R′ and tooth G where measurements of α=3.5° and β=14° have been taken (this situation is not shown in the drawings).